The objective of the proposed experiments is to compare the effect of X-rays and neutrons from the University of Washington cyclotron (21.5 MeV d plus yields Be reaction) on tumor cells growing in vitro, tumors in vivo, and mouse skin. Emphasis will be placed on different responses of cells following X-ray and neutron doses isoeffective for cell survival. Fractionated X-rays only will be compared to mixed neutron-photon fractionation schemes in which neutrons are given on some days and X-rays on others (n-x-x-x-x, n-n-x-x-x, and n-x-x-x-n). Progression delay, reassortment of surviving cells around the cell cycle, and patterns of sublethal and potentially lethal damage repair will be examined in EMT-6 tumor cells in vitro and tumors growing in vivo. Patterns of reoxygenation after iso-effective X-ray and neutron doses will be examined in EMT-6 and KHT mouse sarcomas. The effect of multiple small priming doses of X-rays or neutrons on capacity for sublethal damage repair will be studied in KHT sarcomas. Neutrons plus chemotherapeutic agents will be compared to X-rays plus drugs for effect in killing tumor cells or in producing acute skin damage in mice. Cell cycle kinetics and the structure and function of tumor vasculature will be assessed in EMT-6 sarcomas recurring after X-ray and neutron doses isoeffective for cell killing.